Over the last year, we have started the invasive studies comparing the newly developed open circuit technique to the direct Fick method. During 1998, we performed 4 of these studies during rest through various intensities of exercise, including maximal exercise. The preliminary results of the study have resulted in an abstract submission to the American College of Sports Medicine. Despite the invasive nature of the studies, there have been no problems. Our plans are to test 2 additional subjects, likely completing the studies during the winter and spring of 1999. The results of the study are quite promising and will likely contribute significantly to the non-invasive techniques currently used in the GCRC and may provide additional preliminary data for a subsequent grant submission concerning ventilation and perfusion matching during exercise. Several subjects recruited for the study did not complete all of the sessions for several reasons. These included 1 subject due to pregnancy, 1 due to a recently diagnosed medical concern, 1 subject due to leaving the clinic to perform medical rotations at Scottsdale and 1 subject preferred not to perform the invasive study. Each subject performs an initial maximal exercise test, a test to compare two non-invasive cardiac output techniques and a final test comparing the open circuit technique to the Fick technique.